Condensation process and containment vessel

ABSTRACT

A condenser and a containment vessel adapted to efficiently condense a gas out of a mixture of condensing and non-condensing gases. In condensers, the fraction of gas within a condenser made up of non-condensing gases can be significantly reduced by withdrawing gas from localised regions of relatively low temperature where the mass fraction of non-condensing gases will be relatively high. In containment vessels, pressures can be reduced by providing a large surface area of the liquid into which the condensing gas condenses in a relatively cool region of the containment vessel. Both these effects result from an appreciation of the manner in which non-condensing gases tend to accumulate in regions which are relatively cold.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 10/553,541filed Oct. 26, 2006, now U.S. Pat. No. 7,780,767, which is a §371national stage of PCT/GB2004/001511 filed Apr. 7, 2004, which claimspriority to U.K. Application GB0308657.6 filed Apr. 15, 2003, all ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to condensers and methods for enhancingthe efficiency of condensers. Various embodiments of the invention areapplicable to the condensation of a first gas that is mixed with atleast one second gas having a lower boiling point than the first gas.Various embodiments of the present invention are applicable inparticular to steam condensers of the type used in electric powergeneration, but is also applicable in any circumstances in which a gasis being condensed from a mixture of gases.

BACKGROUND OF THE INVENTION

It has been well known for many years that the rate at which the gas ina mixture of gases condenses is reduced by the presence ofnon-condensing gases in the mixture. The problem has been recognised bymanufacturers of condensers for steam turbines used in electricitygeneration, and by manufacturers of many other types of plant.Condensation heat transfer is however difficult to measure as it is arapid and violent process in which a large quantity of heat istransferred through an interface across which the temperature differenceis intentionally small.

The physics of condensation of a pure gas is well known. Thiscondensation process can best be understood by considering the simplesituation of a flow of pure gas normal to a vertical surface that ismaintained at a constant temperature by a cooling system such that thegas condenses on the vertical surface. Once equilibrium has beenestablished, at every location on the surface a film of condensate formsat a rate that is constant. The condensate runs down the surface undergravity but once equilibrium has been established at any given locationthe film of condensate is of a constant thickness. Substantially, thevariation in temperature in the bulk gas normal to the surface isnegligible, and so the temperature difference between the bulk gas andthe vertical surface is entirely across the thickness of the condensatefilm. These physical processes are sufficiently well understood toenable the design of simple heat exchangers for condensing pure gaseswithout referring to empirical correlations.

It will be appreciated that generally the gas flow towards a cooledsurface in a condensation plant is not all normal to a plane verticalsurface. A common arrangement is a “shell and tube” structure which hasa “nest” of parallel horizontal tubes through each of which tubes acooling liquid is passed. Gas condenses on the cold outer surfaces ofthe tubes and droplets of condensate fall from the tubes. Some of thesedroplets land on lower tubes, thereby increasing the thickness of thefilm of condensate on those tubes and increasing the resistance to heattransfer. This process, called inundation, complicates the estimation ofthe performance of such condensers and accordingly empiricalcorrelations are generally used to predict the overall performance ofsuch systems.

Whereas the physics of condensation of pure gases is relatively simple,the physics of condensation of a mixture of gases is considerably morecomplicated. The energy released per unit of gas condensed is of coursethe same for mixtures as for pure gases, but the rate at which energycan be transferred between the mixture of gases and the condensingsurface is significantly reduced by the presence of the non-condensinggas or gases.

As in the case of a pure gas, when equilibrium is established, gascondenses to form a film of condensate on the heat exchange surface at arate that is constant. Condensate runs down or falls off the surface atthe same rate as it forms, and in the case of a vertical cooling surfacea film of condensate is formed which is of constant thickness at anygiven location. It has been found however that the concentration of thenon-condensing gas or gases increases in a boundary layer of gas betweenthe bulk gas and the condensate film. This increase in the concentrationof non-condensing gas results from the fact that non-condensing gas isswept towards the cooling surface with the condensing gas andaccumulates adjacent the cooling surface. This increases thenon-condensing gas mass fraction in the boundary layer, which restrictsthe flow of the condensing gas towards the condensing surface.

The problems that arise when condensing a first gas from a mixture ofgases as a result of the presence of non-condensing gases have beenappreciated for many years. It appears however that the practicalsignificance of this problem has been underestimated. To assistunderstanding of this problem, the impact of the problem on condensingsteam from a mixture of steam and air is discussed below.

If a mixture of steam and air, at the saturation temperature of thesteam, flows normal to a plane vertical cooled surface maintained at aconstant temperature below the boiling point of water, when the steamand air mixture arrives at the cooled surface steam condenses to form afilm of water on that surface. Air in the mixture however remains in theform of molecules of its constituent gases. The presence of this airimpedes the flow of more steam molecules to the surface of the waterfilm. The rate at which air can diffuse back against the flow of steamlimits the rate at which steam can flow towards the cooled surface. Thusa boundary layer forms in which the air mass fraction increases from asubstantially uniform generally low value in the bulk gas to asubstantially higher value at the surface of the water film. Given thatthe pressure of the gas mixture everywhere in the condenser is the sumof the local partial pressures of the steam and the air and that theflow distance through the boundary layer from the bulk gas to thecondensate film is small, the total pressure at both sides of theboundary layer must be substantially the same. At the surface of thecondensed water film, the steam must be at saturation temperature. Giventhat the air partial pressure at the surface of the water has increased,the steam partial pressure there has reduced. Therefore the saturationtemperature of the steam there is also reduced, and a temperaturedifference has developed across the boundary layer.

Generally an equilibrium situation is quickly established. Thus,although with the condensation of a pure gas the variation intemperature of the bulk gas normal to the cooled surface is negligible,and the bulk gas in effect extends up to the surface of the condensatefilm, with a steam/air gas mixture there is a temperature differencebetween the bulk gas and the surface of the water film that hascondensed out of the mixture. The total temperature difference betweenthe bulk gas and the cooled surface upon which steam condenses isdropped across the total width of the boundary layer plus the condensatefilm, but the temperature difference across the boundary layer issubstantially more significant than the temperature difference acrossthe condensate film.

The above problem has been discussed in the context of a plane verticalcooling surface, but generally condensers do not have such a simplestructure and therefore inundation further complicates the estimation ofthe performance of the plants. As a result many empirical correlationsare generally necessary during the design of condenser plant. Althoughthe problems encountered in condensing one gas of a mixture of gases hasbeen described in the context of steam/air mixtures, it will beappreciated that other gas mixtures behave in a similar way.

The effects of non-condensing gas in condensers are, of course, reducedif a flow of the mixture is imposed parallel to the heat exchangesurfaces. This reduces the concentration of the non-condensing gas orgases adjacent to the heat exchange surfaces. Generally the knownproblems associated with non-condensing gases in condensers have beenaddressed by imposing flows relative to the heat exchange surfaces.Nevertheless, although the principles of the processes have beenunderstood for some time, it has not been possible readily to predictthe rate at which the processes proceed with accuracy and therefore, inpractical condensing plant, it has been conventional to rely uponempirical correlations when estimating the surface area required for acondenser heat exchanger. Of course, a difficulty when relying uponempirical correlations is that it is not easy to predict the performanceof a “perfect” plant so the extent of any short fall of performance maynot be recognised. Furthermore, if any of the correlations relied uponare not quite correct, the overall design cannot be optimum. Furtherdetails of the effects of non-condensing gas on condensation processesare given in “Convective Boiling and Condensation”, chapter 10, J GCoilier, ISBN 0-07-011798-5. This includes a discussion of condensationin the present of a non-condensing gas and reports calculations of theeffects of the non-condensing gas on reducing the heat transfercoefficient for both no imposed flow (also known as free convection),and for an imposed flow (also known as forced convection). “Condensationof a Vapour in the Presence of a Non-conducting Gas”, J W Rose, Int JHeat Mass Transfer Vol. 12 pp 233-237 1969 compares the results of asimplified calculation method with the results for free convection.

An experiment measuring the heat transfer of interest is described in“Measurements of Condensation Heat Transfer Using a Variable ConductanceHeat Pipe”, J A Robinson et al, Second UK National Heat TransferConference, ImechE 1988. This paper describes a technique of measurementof the effect of a mixture of steam and air condensing on a verticalthin disc. Some measurements of heat transfer coefficient at differentair mass fractions are plotted. The anticipated effect of an imposedvelocity parallel to the condensing surface on some reduction of thedependence of the heat transfer coefficient on air mass fraction is alsoshown.

SUMMARY OF THE INVENTION

Some embodiments of the present invention are based on the theorydiscussed above and the realisation that the effect of non-condensinggases on condensation processes has been generally under-estimated,enabling surprising improvements in performance to be achieved byrelatively modest modifications to current design thinking. Someembodiments of the present invention have as one of their objects thedelivery of such performance improvements.

According to another embodiment of the present invention, there isprovided a method for removing non-condensing gas from a mixture ofcondensing and non-condensing gases in a condenser, wherein gas iswithdrawn from at least one location within the condenser, the locationbeing selected to correspond to a region within the condenser in whichthe gas is at a temperature which is lower than the temperature of gasin other regions within the condenser.

Yet another embodiment of the present invention also provides acondenser for condensing gas in which gas is condensed to liquid on aheat exchanging surface, comprising means for withdrawing gas fromwithin the condenser to remove non-condensing gas, the gas withdrawingmeans being positioned to withdraw gas from at least one location inwhich the gas temperature is lower than in other regions within thecondenser.

The above first aspect of the present invention may include therealisation that non-condensing gases tend to accumulate in regions of acondenser which are relatively cold and therefore extracting gas fromsuch regions results in a relatively efficient removal of non-condensinggases from the condenser.

Gas may be extracted from adjacent an arrangement designed to produce alocalised region of relatively cold gas. For example, the arrangementmay comprise a structure positioned so as to be cooled by condensingliquid. The structure may be in the form of a deflector located beneaththe heat exchanging surface such that droplets of condensate fall ontoand cool the deflector, gas being withdrawn from beneath the deflector.For example the deflector could be a simple cover extending over anupwardly extending gas withdrawal pipe, or an elongate gas withdrawalduct a lower side of which defines apertures through which gas is drawninto the duct. As a further alternative, the deflector may be a simpleelongate duct an underside of which defines an open channel, gas beingwithdrawn from one end of the duct. The elongate duct may extend beneathand preferably in parallel with a heat exchanger tube of the condenser.A shield may be located above the deflector to shield falling dropletsof condensate from gas flowing through the condenser, thereby preventingthe gas flow reheating falling droplets of condensate before they strikethe deflector.

In one alternative arrangement, a surface is defined within thecondenser which is cooled by an external means, for example a flow ofcoolant in thermal contact with that surface. The surface could becooled to a temperature lower than any other surface in the condenser.For example, primary and secondary heat exchangers could be provided inseries in the flow of gas to be condensed, the secondary heat exchangerbeing colder than the primary heat exchanger to deliver relatively coldcondensate. Condensate withdrawn from the condenser could be passedthrough an auxiliary heat exchanger in the condenser to heat thecondensate. The cooled surface may be defined by the surface of a poolof condensed liquid in thermal contact with a cooling device. Thecooling device could for example be immersed in the condensate pool.

Alternatively, the cooled surface could be defined by a wall of thecondenser in thermal contact with a cooling device. For example thecondenser wall could be defined by a cover plate which covers anaperture in the condenser, gas being withdrawn through the cover plate.The pressure and temperature of gas adjacent the cover plate could bemonitored, the degree of cooling applied to the plate being controlledto maintain the temperature of the cover plate above the freezing pointof the condensed liquid.

According to yet another aspect of some embodiments of the presentinvention, there is provided a method for establishing favourabletemperature differences between heat exchanger conduits within acondenser and a process fluid which flows through the condenser, whereincoolant is pumped through an array of parallel heat exchanger conduitsspaced apart in the direction of process fluid flow, at least two of theconduits being connected in series such that coolant flows sequentiallythrough first and second conduits, the second conduit being locatedupstream of the first conduit in the direction of process fluid flow.

A further aspect of some embodiments of the present invention provide acondenser comprising an array of parallel heat exchanger conduits spacedapart in the direction of flow of a process fluid flow including a gasto be condensed, wherein at least two conduits that are spaced apart inthe direction of fluid flow are connected in series such that coolantflows sequentially through first and second conduits, the second conduitbeing located upstream of the first conduit in the direction of processfluid flow.

A first pair of first and second conduits may be connected in series, asecond pair of first and second conduits may be connected in series, thedirection of flow of coolant through the condenser being in onedirection for the first conduit of the first pair and the second conduitof the second pair and in the opposite direction for the second conduitof the first pair and the first conduit of the second pair, the secondconduit of the first pair being located upstream in the process flow ofthe first conduit of the second pair, and the second conduit of thesecond pair being located upstream in the process flow of the firstconduit of the first pair.

The parallel heat exchanger conduits may comprise parallel heatexchanger conduits.

Alternatively, the parallel heat exchanger conduits may be defined by astaggered array of baffles. Each baffle extends transverse the directionof flow of the process fluid, with alternate baffles extending fromopposite sides of the condenser. The condenser further comprises anarray of process fluid tubes extending through the baffles for said flowof the process fluid.

According to yet another aspect of embodiments of the present invention,there is provided a method for minimising the pressure within acontainment vessel resulting from the release into the vessel of apressurised gas which will condense to a liquid at the temperatures andpressures assumed to prevail within the containment vessel, wherein abody of the liquid of large surface area relative to the area of thevessel is established in a lower portion of the vessel.

In some embodiments this aspect of the present invention also provides acontainment vessel intended to contain a release into the vessel ofpressurised gas which will condense to a liquid at the temperatures andpressures assumed to prevail within the containment vessel, thecontainment vessel initially being filled with a gas or gases which willnot condense at the temperatures and pressures assumed to prevail withinthe containment vessel, and the containment vessel including means forestablishing in a lower portion of the vessel a body of the liquid oflarge surface area relative to the area of the vessel.

The body of liquid may be established in a simple open tray arranged tocollect condensing liquid. Alternatively, means may be provided forreleasing a stored volume of the liquid into at least one open tray toform the body of liquid. Pressure within the containment vessel may besensed, the stored volume of liquid being released in the event of thesensed pressure exceeding a predetermined threshold. These and otheraspects of various embodiments of the present invention will be shown inthe text, drawings, and claims that follow.

DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings, in which:

FIGS. 1 to 3 present information derived from the theory discussed aboveillustrating the effect of non-condensing gases on condensationprocesses, FIG. 1 showing the relationship between heat transfercoefficient and air mass fraction for a mixture of steam and aircondensing on a vertical thin disc, FIG. 2 showing the relationshipbetween heat transfer coefficient and air mass fraction at reducedpressures, and FIG. 3 showing the relationship between the fraction ofthe temperature difference between the bulk steam/air mixture and thecooled surface which arises across the condensate film;

FIG. 4 presents information discussed in the above paper by Robinson,showing the relationship between heat transfer coefficient and air massfraction for increasing forced convection velocity;

FIG. 5 is a schematic representation of the relationship betweentemperature and entropy describing the closed thermal cycle of a steampower station with a turbine generator;

FIG. 6 represents a vertical section through a known steam turbinecondenser;

FIG. 7 represents the same structure as FIG. 6 but taken through avertical section perpendicular to that of FIG. 6;

FIG. 8 represents the position of air extract devices according to oneembodiment of the present invention in a structure such as thatillustrated in FIGS. 6 and 7;

FIGS. 9A to 9E represent alternative embodiments of the presentinvention of air extract devices to that shown in FIG. 8;

FIGS. 10A and 10B represent a shielded air extract gas intake accordingto yet another embodiment of the present invention which may be used asan alternative to that shown in FIG. 8;

FIG. 11 schematically illustrates the removal of non-condensing gasesaccording to yet another embodiment of the present invention by relyingupon localised cooling of the gases;

FIG. 12 schematically represents the overall structure of a modifiedsteam condensing plant according to another embodiment of the presentinvention;

FIG. 13 represents an alternative arrangement to that shown in FIG. 12;

FIG. 14 schematically represents a further embodiment of the presentinvention having an arrangement for achieving a double pass of coolantthrough a condenser;

FIG. 15 schematically represents a further embodiment of the presentinvention having an alternative double pass design to that illustratedin FIG. 14;

FIG. 16 represents the application of a still further embodiment of thepresent invention to a containment vessel such as that used in a nuclearpower station; and

FIG. 17 schematically represents the overall structure of a modifiedsteam condensing plant utilising vertical tubes in another embodiment ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations and furthermodifications in the illustrated device, and such further applicationsof the principles of the invention as illustrated therein beingcontemplated as would normally occur to one skilled in the art to whichthe invention relates.

Referring to FIGS. 1 to 3, these present information derived from theaccepted theory as discussed above and illustrating the effect ofnon-condensing gases on condensation processes. From FIG. 1 it can beappreciated that the heat transfer coefficient falls rapidly withincreasing air mass fraction in a boundary layer between bulk gas and afilm of condensate. FIG. 2 shows that the reduction in the heat transfercoefficient is more severe at lower pressures. FIG. 3 shows that atvalues of air mass fraction which will be representative of conditionsin a steam condenser the proportion of the temperature differencebetween the bulk steam/air gas and the cooled surface upon which steamis condensing rises rapidly within increasing air mass fraction suchthat at representative values a large proportion of the temperaturedifference arises across the boundary layer of relatively high air massfraction adjacent the condensate film.

FIG. 4 presents information showing that an imposed gas velocityparallel to the condensing surface improves the heat transfercoefficient. But this is still expected to be significantly affectedeven at relatively high imposed velocities.

Some embodiments of the present invention makes it possible to reducethe air mass fraction immediately adjacent the cooling surfaces of heatexchangers and as a result to improve the efficiency of such devices.This makes it possible to reduce the size of a condenser for aparticular purpose as compared with conventionally designed condensersused for that purpose. Overall efficiency of plant is also increased.The result is potentially of very significant commercial importance.

By way of illustrating the principle behind some embodiments of thepresent invention, a vertical downward flow of a steam-air mixturepassing a horizontal tube in which a coolant flows may be considered. Ifthe coolant at the hot end of the tube is 20° C., to a goodapproximation this will be the temperature of the water condensed atthat location and therefore also the temperature of steam condensing atthat location. Thus the partial pressure of the steam at that locationwill be the saturation pressure given a temperature of 20° C., that is23 mbars. If the coolant at the cold end of the tube is 10° C., then bythe same argument the partial pressure of steam at that location will be12 mbars. The total pressure at both locations will be the same, thebalance being made up by the partial pressure of air at each of thelocations.

If it was possible to remove some of the air from around the boundarylayer adjacent the cold end of the tube, flow and diffusion processeswill also reduce the air mass fraction elsewhere in the condenserincluding the hot end of the tube. Thus the partial pressure of air willbe reduced at both locations, and thus the pressure of the bulk gasaround the tube will also be reduced. There is however a further effect,that is the reduction in the air mass fraction at the interface betweenthe bulk gas and the tube will reduce the temperature difference acrossthe boundary layer around the film of water on the tube. As thetemperature of the tube is assumed to be substantially constant, thetemperature of the bulk gas will be decreased. Therefore the partialpressure of the steam in the bulk gas will also be decreased. Thisresults in a further reduction in the pressure of the bulk gas.

Thus, extracting air from where its concentration is highest, that isadjacent the cold end of the tube, reduces its partial pressurethroughout the volume of the condenser. This improves the heat transferprocess, resulting in the condensation of more steam and a reduction inthe partial pressure of steam throughout the volume.

It will be appreciated that there will be some slight temperaturedifference across the film of water formed on the tube but this will berelatively insignificant and will not affect the overall situation asdiscussed above.

If the tube is a tube of a condenser as used in electric powergeneration, the function of the condenser is to remove the latent heatfrom steam after it exits a turbine so that the steam is condensed towater. The coolant flowing through the tubes is usually water taken froma separate and dedicated system, whereas the process flow is steam.After expanding from high pressure, steam at an outlet of a turbine isat a sub-atmospheric pressure. As a result the pressure within thecondenser volume is lower than the pressure around the condenser andtherefore air will leak into the process flow through seals in theturbine and in the body of the condenser. The structure of the condenseris generally large, typically in excess of 6 metres high 15 metres longand many metres wide. Given the size of such structures and the factthat they are exposed to fluctuating temperatures it is generally notpossible to design them in a way that will fully eliminate the leakageof air into the structure when the interior structure is subjected to apartial vacuum. Furthermore, when first installed or after maintenancethe structure is in any event full of air at atmospheric pressure.Elimination of air from the structure is therefore not a practicalpossibility. Generally condensers of this type define an enclosurehousing a nest of tubes through which cooling water flows. The tubes arearranged in parallel so as to form a central open region about which thetubes are distributed. An air extraction duct is in communication withthe central open region and air and steam within the central open regionis simply pumped out of the condenser enclosure.

Typically, in condensers for a large steam turbine generatingelectricity of the type described, operational parameters are asfollows:

1. the inlet coolant water temperature is 10° C.;

2. the outlet coolant water temperature is 20° C.;

3. the pressure at the outlet of the turbine which communicates with thecondenser enclosure is 50 mbar;

4. the temperature of condensate at outlet, which formed on the tubesand then fell to the bottom of the condenser enclosure, is 33° C.;

5. the flow rate of steam is 360 kg/s;

6. the flow rate through the air extraction system is 130 kg/h.

It will be appreciated from the above that the extract flow rate is onepart in 10,000 of the steam flow. The temperature of the condensate is,within measurement error, the saturation temperature of steam at 50mbars. Thus it is not at all obvious that condensate forms at a lowertemperature and that it is possible to reduce the vacuum after exit fromthe turbine. Indeed, the set of parameters outlined above would be takenas indicating an excellent condenser design, as both the vacuum is inline with conventional expectations and the condensate has not beencooled to an unnecessarily low temperature. Although what has been shownand described are operation parameters for one type of condenser, thepresent invention is not so constrained, and contemplates application toany type of condenser.

It will be appreciated that in the described structure water dropletsare cooled to a temperature of between 10° C. and 20° C. and then heatedup to 33° C. as they fall through the bulk gas within the condenserbody. Air is pulled through the nest of tubes, degrading the heattransfer of every tube, with the degradation increasing towards the opencentral region of the nest of tubes. If it was the case that all of theextracted flow was air this would imply that the value of the air massfraction within the condenser is one part in 10,000, but given theunderstanding of the physics as represented in FIGS. 1 to 4, it will beappreciated that the value of the air mass fraction at the inner surfaceof the boundary layers formed around the tubes will be very much higherthan one part in 10,000. Furthermore, the air mass fraction within thecentral region of the nest of tubes will also be higher than one part in10,000. Typically the flow rate of steam that is not condensed andtherefore is extracted with the residual air is about the same as thatof the extracted air, and so the air mass fraction of the extracted flowwill be around 0.5. Where there are water droplets at temperaturesbetween 10° C. and 20° C., the air mass fraction may be higher than thatat the extract from the central region. Some air will remain in coldregions of the condenser that are not swept by the flow and thus thepartial pressure of the air in such regions will not be particularly lowand is a potential source of instability as operating conditions change.

Given that a significant proportion of the extracted gas is steam, thereis a continuous loss of process gas and therefore of potentialcondensate. In this type of industrial process this causes a requirementfor a make-up flow, which in other types of process would result in aloss of potential product. Furthermore, energy is being supplied toextract this flow of potential condensate which represents a waste ofenergy. Energy loss can also arise as a result of heating of the extractpipe where it traverses through the bulk gas in the central region ofthe nest of tubes.

Although the presence of a non-condensing gas degrades the rate ofcondensation of a gas condensing out of a mix of gases, there is littleunderstanding of the sensitivity of the process to even small traces ofnon-condensing gases. Although it has been appreciated that imposing aflow across condensing surfaces reduces the effects of non-condensinggases the underlying significance of the presence of traces ofnon-condensing gases has not been appreciated. Problems have arisen inthe past as a result of the presence of non-condensing gas incondensers. For example, in the 1960's and 1970's a number of steamturbine condensers were installed in the United Kingdom. These did notperform as well has had been expected and as a result an extensiveresearch programme was initiated. It was found that regions of the tubenest within the condensers were being blanketed by air, which locallyreduce the flow of steam. Modifications were made to the tube nests, andas a result the plants performed more or less as expected. Accordinglyit was believed that the degradation of performance by the entrained airhad been understood and solved. This was not the case.

Referring to FIG. 5, this represents the well-known thermal cycle of asteam power station with turbine generator. The temperature of the heatsource of the system is represented by a broken line 1 and thetemperature of a heat sink, for example a source of cold water, isrepresented by broken line 2. The temperature rise represented by theline between points 3 and 4 corresponds to an increase in pressurebetween a condenser output and the input to the water heating system.The line between points 4 and 5 corresponding to heating of the water.The line between points 5 and 6 corresponds to the formation of steam ata stable temperature corresponding to the boiling point of water. Theline between points 6 and 7 corresponding to super-heating of thepreviously formed steam. The line between points 7 and 8 a correspondsto the extraction of energy from the super-heated steam by the driventurbine. (The line between points 6 and 8 b corresponds to thealternative of extraction of energy from saturated steam).

The line between points 8 a and 3 (or 8 b and 3) corresponds to theresidual heat rejected to the heat sink. The spacing between the linefrom points 8 a or 8 b to 3 and broken line 2 corresponds to thedifference between the temperature of the gas in the condenser and thetemperature of the coolant. The smaller this difference can be made, thegreater will be the efficiency of the generating system as the steamdelivered to the turbine will expand to a lower pressure, therebygenerating more electricity and increasing the efficiency of the cycle.The present invention enables this temperature difference to be reduced.

Referring now to FIGS. 6 and 7, the structure of a known steam condenseris illustrated in general terms. Generally such condensers comprise anest of horizontally extending tubes which carry cooling water, steam tobe condensed being arranged to flow inwards from the exterior of thenest to an open region at the centre. In FIGS. 6 and 7, broken lines 9represents the outer periphery of the nest of tubes, and broken lines 10the inner limit of the nest of tubes. The central open region isrepresented by the area 11. Steam flowing radially inwards isrepresented by arrows 12. Flow 13 in FIGS. 6 and 7 represent theoff-take for air and the residue of steam that has not condensed. Acondensate collection system 14 collects falling water droplets, theflow of condensate being extracted via outlet 15. One of the nestedtubes is represented by component 16 in FIG. 7.

As discussed above, it is generally assumed that, because the air massfraction at the bulk gas inlet (the outside of the tube nest) is so low,and there appears to be little air blanketing, the air in the structurehas little effect. However this is not the case. Locating the airextract system on the downstream side of the tube nest and in the openregion in the centre results in a large air-rich volume within thecondenser. Furthermore the inward flow of air, of progressivelyincreasing air mass fraction, “poisons” the condensation heat transferprocesses on every tube of the condenser.

FIG. 7 indicates that the pipe work for the extract flow 13 for theknown steam condenser has to pass through the bulk gas. The gas flowingthrough this pipe work contains a significant proportion of gas thatcould potentially be condensed. Thus the pipe work, pump and fittingshave to be sized appropriately. This is expensive. The pump consumesenergy. There is a need to make up for the loss of potential condensate(and in other processes there will be a loss of potential product).These costs can be minimised by thermally isolating or insulating thisextract system from the effects of the bulk gas to prevent condensatefrom being re-evaporated and the extracted gas being re-heated. Botheffects will increase the volume flow rate of gas, which increases thebackpressure, thus offsetting some potential benefits of the design.

Droplets of water falling off the condenser tubes are at approximatelythe same temperature as the surface of the tubes. Therefore, adjacent towhere these droplets are formed, the air mass fraction is high. As thedroplets fall through the local steam/air mixture they are heated up tothe temperature of that mixture, and the air mass fraction reduces.Accordingly, if gas is removed from the structure at locations where thegas mixture is still relatively cool, the proportion of air in theremoved gas is relatively high.

Referring to FIG. 8, this represents the disposition of an array of thetubes in the tube nest in the bottom portion of the structure shown inFIG. 7 according to one embodiment of the present invention. Tube 17represents the position of a tube adjacent the open interior of thestructure, whereas tube 18 represents the lowermost tube in thestructure. Cold coolant is introduced to the left hand end of thesetubes (in the example above at a temperature of 10° C.) as indicated byarrows 19, and the heated coolant (in the example above water at atemperature of 20° C.) issues from these tubes as represented by arrows20. Immediately adjacent the inlet end of the tubes 17 and 18 airextract ducts 21 are provided, the inlet ends of these ducts beinglocated beneath droplet deflectors 22 arranged to prevent any dropletsof water entering the air extract ducts 21. The uppermost duct 21 isplaced adjacent the interior of the tube nest. (The flow of steam isperpendicular to the axis of the tube as represented by arrow 23). Thelowermost duct 21 is located at the bottom of the structure given thatthere is a tendency for gas incorporating a large volume of air to fallto the bottom of the structure. Thus, by the simple expedient offittings ducts 21 as shown in FIG. 8 the proportion of the air adjacentthe tubes of the condenser can be significantly reduced.

One effect from the limited number of air extract ducts as shown in FIG.8 is achieved by locating them adjacent the “cold” end of the tubes ofthe condenser. Some benefit may, however, be achieved by providing ductsextracting air from points distributed along the length of the tubes asrepresented by broken lines 24 and 25.

FIG. 8 represents a single vertical section through the tube structureand accordingly it will be appreciated that ducts 21 will be distributedin a direction perpendicular to the plane of FIG. 8.

It will be appreciated that any convenient structure could be providedto prevent condensate being extracted through the air extract duct 21.The simple “roof” structure shown in FIG. 8 could be substituted forexample by a pipe with orifices in the bottom as shown in FIGS. 9A and9B, FIG. 9B being a section on line 9B to 9B of FIG. 9A, or by a memberof inverted U-shaped section as shown in FIGS. 9C and 9D, FIG. 9D beinga section on line 9D-9D of FIG. 9C or by a rectangular channel with aslot cut in its base or the like. As illustrated in FIGS. 9E, an arrayof interconnected extract pipes 21 could extend perpendicular to thetubes 18.

A shield could be provided to protect the falling droplets of coldcondensate and surrounding cold gas from the temperature and velocity ofthe hotter flow of the bulk gas, which would reduce the mass fraction ofnon-condensing gas. FIGS. 10A and 10B represent such a shield, the sideelevation of FIG. 10A showing the axial length of the shield 26 is shortto protect the gas extract. The shield 26 is generally cylindrical, withits lower edge surrounding the deflector 22 and its upper edge extendingto adjacent the tube 17. The end elevation of FIG. 10B shows theclearance around the coolant tube and deflector is small.

Thus, air is extracted from the coldest points within the structure. Theappropriate positions of air extract ducts can be determined bymeasurement of the temperatures of the structure before modification. Itwill of course be appreciated that falling droplets of condensate createa localised volume of high air mass fraction which makes an arrangementsuch as that illustrated in FIG. 8 particularly appropriate. The airextract ducts reduce the concentration and partial pressure of air inthe localised volumes of highest concentration, and then diffusionprocesses reduce the concentration of air throughout the structure.Hence, even if air is extracted from only a limited number of regions ofthe structure, the heat transfer processes of the condenser will beimproved throughout the structure.

Referring to the structure of a known condenser in which the air isextracted from the centre of the tube nest, care should be taken in someembodiments to ensure that the extract flow of gas is taken from thecoldest part of the volume. Measures such as the use of a suitable diptube (perhaps thermally insulated) hung from the structure above wouldminimise the transfer of heat into this volume and so maximise the airmass fraction in the extract flow and minimise process flow losses. Inparticular the contents of the extract ducting should be thermallyinsulated from the inlet steam/air mixture from the turbine.

An auxiliary cooling system, or sub-condenser, could be installed tocreate localised volumes of very high air mass fraction, and additionalair extract points located appropriately. For example, in generatingplant where cooling towers form the main heat sink, make-up water couldbe colder than water from the cooling towers and would provide anappropriate heat sink. Alternatively a dedicated supply could beinstalled. An effective approach would be to install a plant to freezeout the remaining potential condensate. One approach would be to utilisecovers used during construction or for inspection. FIG. 11 illustrates amodified cover provided for this purpose.

Referring to FIG. 11, the condensate pressure vessel has a wall 27defining an aperture covered by a plate 28 insulated from the wall 27 bya thermally insulating gasket 29. The plate 28 defines many smalldiameter passages (only two of which are shown) communicating with amanifold connected by a suitable pipe 30 to a pump 31 and a flow meter32. Cooling and heating coils (not shown) are incorporated in a body 33in good thermal contact with the plate 28. A pressure sensor and atemperature sensor sense the pressure adjacent the plate 28 and thetemperature of the plate 28.

In use, the cooling and heating coils would be controlled to maintainthe temperature of the centre of the plate 28 at just above the freezingpoint of water given the sensed pressure, thereby maximising the airfraction in the condenser adjacent the plate 28 and therefore maximisingthe air fraction in the extracted flow. If the flow-meter was to detectzero flow and the sensed temperature indicates there could be frozenwater in the extract system, the control system would reduce the coolingair or halt cooling and turn on the heating coils.

In the arrangement illustrated in FIG. 11, a localised “cool spot” isproduced by cooling a modified cover plate adjacent which a relativelyhigh concentration of air accumulates. Alternative approaches may beadopted to produce a localised cool spot. For example, in a condenser inwhich a condensate pool forms all or part of the condensate pool coulditself be cooled so as to encourage concentration of air adjacent thepool. Gas would then be extracted from adjacent the condensate pool.

Although in the above example embodiments of the present invention hasbeen described with reference to a particular design of condenser forsteam generating plant, it will be appreciated that the invention canalso be applied to other designs of steam generating plant condensersand to other industries. For example, the performance of chemical andoil refinery processes could be improved by localised concentration of agas to be extracted. Steam sterilisation plant for medical use and foruse in the food industry could also be made more efficient, resulting inreduced cycle time to establish an initial vacuum and more reliablecontrol of temperatures during a sterilisation process.

The embodiments of the invention described above are immediatelyapplicable in existing condensers which are in use in steam generatingplant. It is envisaged that very substantial increases in efficiency byretrofitting the invention to existing plant can be achieved. Theinvention can be incorporated in new plant however and it is envisagedthat in for example condensers for steam generating equipment theapplication of the invention will enable a dramatic reduction in thesize of condenser for a particular application. For example it isenvisaged that condenser sizes may be reduced by a factor of as much asfive. It will be appreciated that this will represent a radicaldeparture from existing design assumptions.

For example, again using the condenser of a turbine of steam generatingplant as an example, instead of having a nest of tubes with gas flowsinwards towards the centre, with air being extracted from the centre, adesign could be envisaged which would have a straight-through flow. Theterm straight-through flow is used to indicate a condenser in whichsteam with entrained air enters the condenser through an upper end ofcondenser housing, air being extracted from suitable locations whichcorrespond to localised cool spots resulting from for example fallingdroplets of condensate or at the bottom of the structure just above acondensate pool. In the latter case, the area of the condensate poolshould be as large as possible. The condensate pool could incorporateits own heat exchanger, perhaps cooled by a flow of coolant at thecoolant inlet temperature of the condenser. This would offset anypotential for incoming steam to evaporate condensate. The residual ofthe vertical downwards flow of steam would impinge on the condensate inthe condensate pool and some of the residual steam would condense. Afurther advantage of such a condenser geometry is that the bulk flow ofgas is always in the same (vertical) direction, emitting energy lossesdue to changes in the direction of flow within the condenser. Thepressure at the outlet from the turbine could thereby be furtherreduced.

The principles of a straight-through condenser referred to above couldbe embodied in a condenser design such as that illustrated in FIG. 12.In FIG. 12, an incoming air and steam flow is represented by arrow 34.The incoming flow passes through a condenser having an outer body 35 andcondensate is collected in a condensate pool 36. The condensate pool 36has a large free surface area. A heat exchanger 37 is in contact withthe condensate pool 36 so as to cool condensate within the pool. Theresult is a concentration of air immediately above the free surface ofthe condensate in the pool. A nest of horizontal tubes represented byarea 38 is located above the condensate pool, condensing out waterdripping from those tubes into the condensate pool 36. An auxiliarycooling device 39 may also be provided to produce a further cool spot ina manner analogous to that described with reference to FIG. 11. Theauxiliary cooling device 39 could be for example mounted on a removableplate. An outlet (not shown) for non-condensing gas could be provided inassociation with the auxiliary cooling device 39. A non-condensing gasoutlet 40 is provided, the outlet extending from beneath a dropletdeflector 41 similar to the deflector 22 of FIG. 10. The outlet 40 islocated just above the surface of the condensate pool 36 so as to removethe gas of high air content which accumulated immediately above thecondensate.

Condensate will be removed from the condensate pool 36 as indicated byflow 42. That flow may be passed through an auxiliary heat exchanger 42a located at the top of the condenser so as to reheat the condensate byabsorbing energy from the incoming flow of steam and air if condensatereheat is required in a particular application.

A conventional condenser design in which the bulk gas flows inwards hasthe advantage that the cross-sectional area of the flow reduces from theinlet to the outlet, thereby partially offsetting the reducing gasvelocity which results from the reducing mass flow as gas is condensedout of the mixture. This reduction in cross-sectional flow area to anextent mitigates the adverse effects on the condensation process of theincreasing mass fraction of the non-condensing gas. A straight-throughdesign such as that illustrated in FIG. 12 does not provide theadvantages of an inwards flow but nevertheless careful location of thevarious extraction devices through which non-condensing gases areremoved from the condenser still delivers a considerable overallbenefit.

It will be appreciated that the design of a turbine/condensercombination will be selected with a view to optimising performance giventhe conditions that apply in the particular application, for example independence upon the source of cooling water from the sea, a river or alake, the use of wet or dry cooling towers as the final heat sink etc.The designer will have to make a choice between the minimum cost ofcondenser (by minimising the size and the area of the heat transfersurface) or accepting a larger condenser to enable the use of cheapercomponents or design of heat sink to reduce the overall capital cost.The final choice will depend therefore on the site and the economicrequirements, for example whether or not the lowest possible capitalcost or the lowest possible long term running cost is required.

FIG. 13 shows a refinement of the design outlined in FIG. 12 in whichthe main tube nest which is shown as a unitary assembly 38 in FIG. 12 issplit into a pair of in-line sections 43, 44. It will be appreciated ofcourse that more than two in-line sections could be used in an assemblyof the general type illustrated in FIG. 13. Where appropriate the samereference numerals are used in FIG. 13 as in FIG. 12.

The tube nest sections 43 and 44 of FIG. 13 could define separateprimary and secondary heat exchangers with separate heat sinks, thesecondary heat exchanger 44 being supplied with coolant at a temperaturewhich is lower than the temperature of coolant supply to the primaryheat exchanger 43. Condensate dripping off the relatively cold secondaryheat exchanger 44 will be colder than condensate falling off the primaryheat exchanger 43, enhancing the concentration of cold gas of high aircontent in the region from which gas is withdrawn from the condenserbody 35. The lower secondary heat exchanger 44 will reject residual heatand could be designed so that only a small proportion of the total steamcondensed would be condensed within the tube nest 44. In this way thedual functions of the conventional heat sink of rejecting the total heatto the environment, and providing the lowest possible temperature (andtherefore exhaust pressure) can be separated. This increases the optionsfor the choice of heat sink (source of cold water for example) or typeof cooling tower (wet or dry) or makes a combination of different heatsinks and cooling towers a possibility.

Given a straight-through condenser structure as illustrated in FIGS. 12and 13, and assuming that the or each tube nest incorporates horizontalcondensing tubes, various approaches are possible to maximise the dutyof all the tubes. FIG. 14 schematically illustrates a simple arrangementin which the flow of cooling water can make more than one pass throughthe condenser. Such an arrangement can reduce the temperature differencebetween the gas mixture at entry and the first tubes encountered by themixture towards the top of the condenser whilst maintaining an adequatetemperature differential between the gas mixture flow and the tubeswhich are lower down in the tube nest. In FIG. 14, the incoming steamand air mixture is represented by arrow 45, the condenser housing isrepresented by the rectangle 46, and the condensate pool is representedby the shaded area 47. Air is extracted as represented by arrow 48 fromimmediately above the condensate pool and condensate is removed asindicated by arrow 49. The tube nest incorporates an upper tube 50 and alower tube 51 which are connected in series such that coolant enterstube 51 at point A, flows from tube 51 to tube 50 via points B and C,and exits the condenser at point D. Thus the coolant within tube 51 willbe colder than the coolant within tube 50. Given that the steam/airmixture cools as it moves vertically downwards this arrangement ensuresthat the cooling steam/air mixture encounters progressively cooler tubesas it moves vertically downwards.

In the arrangement of FIG. 14, it will be appreciated that the portionof tube 51 adjacent point A can be made to work as efficiently as theend of tube 50 adjacent point D. There will however be no significanttemperature difference between the end adjacent point B of tube 51 andthe end adjacent point C of tube 50.

FIG. 15 illustrates an alternative flow pattern in which a coolantenters a condenser 52 at point A to pass through a tube 53 and exit thecondenser at point B. That flow is then transferred to point C, flowsthrough tube 54 and exits the condenser at point D. A second flow entersthe condenser at point E and traverses the condenser in tube 55, exitsthe condenser at point F, re-enters the condenser at point G andtraverses the condenser through tube 56, leaving the condenser at pointH. Thus in contrast to the arrangement of FIG. 15 where theseries-connected pair of tubes 50 and 51 are parallel and located oneabove the other, in the arrangement of FIG. 15 first and second seriesconnected tube pairs 53, 54 and 55, 56 are spaced apart in both thevertical and horizontal directions. The first flow enters from one sideof the condenser (the side of points A, D, F and G) whereas the otherflow enters from the other side of the condenser (the side of points B,C, E and H). As a result, coolant flows in one direction in the firsttube 53 of the first pair and the second tube 56 of the second pair, butin the opposite direction in the second tube 54 of the first pair andthe first tube 55 of the second pair. The tubes 53 and 56 are spacedapart in the process flow direction, as are the tubes 54 and 55. Thuseach tube can be designed to remove a similar quantity of heat asvertically spaced points on tube pairs 53, 56 and 54, 55 are always atdifferent temperatures. For example it may be that the temperature ofthe coolant at point A will be 10°, at points B and C 15° C., and atpoint D 20° C. Similarly, the temperature at point E may be 10° C., thetemperature at points F and G may be 15° C., with the point H at thefinal exit temperature of 20° C.

It will be appreciated that in the arrangements illustrated in FIGS. 14and 15 extract devices such as those shown in FIGS. 12 and 13 will beprovided to remove non-condensing gas from cold spots within thestructure.

In the embodiments of the invention described above, condenser tubenests are generally described as being made up from a series of parallelhorizontal tubes. Alternative tube arrangements are possible. Forexample, the cooling tubes could be vertical and adopting such a designwould provide an alternative method for maintaining a good temperaturedifference between the gas mixture and the tubes throughout the tubenest. For example, bulk gas could be passed through the condenser in avertically condensing gas flow and coolant could be passed through thecondenser in vertical tubes with the coolant flowing vertically upwardsthrough the tubes. This would improve condensation heat transfer ascompared with conventional designs, and therefore the surface area ofthe tubes making up the nest could be smaller than in current designs.

In systems with horizontal tubes, the coolant temperature varies indirection which is transverse to the vertical component of the flow ofbulk gas and thus alters the heat transfer characteristics across thewidth of the structure. With vertical tubes and a counter-currentvertical flow, the heat transfer characteristics are constant across anyhorizontal plane. The temperature of the coolant at the coolant outletcan therefore be higher and nearer to that of the bulk gas inlet. Thiseffect is integrated throughout the condenser, and so the overall flowof coolant can be reduced, thereby saving pumping power. Some adverseeffects may arise however if it is necessary to dilute the outlet flowwith further coolant to reduce the discharge temperature to the localenvironment.

The upper ends of the vertical tubes could be connected to an outletheader above the tube nest which does not have to be thermally insulatedfrom the bulk gas as its additional cooling effect could be beneficial.An inlet header at the bottom of the tube nest could be immersed in acondensate pool in order to cool the pool, thereby ensuring theaccumulation of non-condensing gases adjacent the cooling pool.Alternatively, the inlet header could be thermally isolated if it wasthought necessary to eliminate condensate sub-cooling in order tomaintain desired control over the entire plant.

In an arrangement with vertical tubes relatively short tube lengths canbe used and differential thermal movements will be reduced. This maymake it possible to have an all-welded structure of coolant tubes. Thusin contrast to existing large scale designs where a large number ofsliding joints are provided to accommodate differential thermal movementdue to temperature changes, such sliding joints could be avoided. Thiswould reduce the risk of leakage. It would also be easier to reduce theleakage of non-condensing gases into the condenser. Sub-assemblies couldbe welded in the factory and shipped to site to be welded togetheron-site. This would reduce the site works, and therefore reduce theconstruction time on-site. It would also enable the start of these civilworks to be delayed, to suit the reduced mechanical site constructionactivities of condenser assembly. The overall cost of the design wouldtherefore be substantially reduced.

In an alternative arrangement with vertical tubes the incoming air andsteam flow is internal to the tubes. FIG. 17 illustrates such anarrangement. An incoming air and steam flow is represented by arrow 34.The incoming flow 34 is incident upon an upper tube plate 65. Tube plate65 is supported by (and sealed to) an annular support 66 extendingaround the interior of the outer body 35. The incoming flow passesthrough an array of vertical tubes 67 forming a single tube nest. Theupper ends of the tubes are sealed to the upper tube plate 65, such thatthe tubes 67 connect with the upper part of the condenser. The lowerends of the tubes 67 are sealed to a lower tube plate 68, such that thetubes 67 connect with the lower part of the condenser allowing processfluid to flow through the tubes. An array of baffles 71 are incorporatedinto the volume between the upper and lower tube plates. These extendpartway transverse the condenser, e.g. across the width of thecondenser. The array of baffles is staggered, with alternate bafflesextending from opposite sides of the condenser. The baffles 71 define anarray of parallel heat exchanger conduits. Coolant is passed into theconduits at a lower inlet 69 and exits via an upper outlet 70. Theconduits extend in series, such that coolant will flow in one directionalong a first of the conduits, and in the opposite direction along thenext, adjacent conduit. Condensate condenses on the inside of tubes 67.Condensate then collects in a condensate pool 36. The condensate pool 36is in contact with a heat exchanger 37 to cool the condensate, asdescribed above in relation to FIG. 12. Similarly a condensate outlet 42is provided.

Non condensing gas outlets 40 are incorporated both in the volume aboveupper tube plate 65 and in the volume below lower tube plate 68.Incorporating a non condensing gas outlet 40 in the volume above uppertube plate 65 allows the counteraction of any leakage of air into thisportion of the condenser through outer body 35. Additionally, this noncondensing gas outlet 40 can also serve to maximise the condensation inthe tubes. Non condensing gas outlet 40 in the volume below the lowertube plate 68 allows air to be removed on start up of the condensationprocess, and the counteraction any leakage of air into this portion ofthe condenser through outer body 35.

Both non condensing gas outlets 40 are shown in conjunction with dropletdeflectors 41 as described above in relation to FIG. 12. It will beappreciated that this outlet design may be replaced by the noncondensing gas outlet of FIG. 11.

This alternative arrangement simplifies the layout for the attachment ofthe upper ends of the tubes. Having the tubes connected via a singletube plate at each end obviates the need to either have multiple tubesexiting through the wall of the condenser, or a complex tube combinerinside the condenser. This also considerably reduces the size of thevolume to be sealed against ingress of air. Only the volume of thecondenser above the upper tube plate 63 needs to be sealed against airingress. As the majority of the condenser is at the temperature of thecoolant rather than that of the air and steam flow, which may bevariable, thermal expansion and contraction of the condenser isminimised.

A disadvantage of a vertical tube design is that the thickness of thecondensate on the cooling tubes will increase with distance down thetubes. This therefore presents a greater resistance to the heat fluxthan for horizontal tube designs. In order to offset this, the tubescould be arranged so as to encourage the formation of droplets so thatthe droplets fall off the tube. For example, the tubes could be arrangedat a small angle from the vertical. Alternatively or in addition thetube surfaces may be shaped to promote break-up of the condensate intodroplets by surface roughening or the addition of short fins. A helicalfin design could be considered.

In straight-through designs such as those illustrated with reference toFIGS. 12 to 15, the velocity of the bulk gas falls with distance fromthe bulk gas inlet. This reduces the velocity of the gas over thesurfaces of the tubes. This makes it even more desirable to reduce thenon-condensing gas mass fraction adjacent the tube surfaces as much aspossible as the effect of the non-condensing gas is compounded by thereduction in gas velocity. The reduction in gas flow does however makeit possible to insert grids or braces into the tube nest so as tosupport the tubes and reduce their tendency to vibrate in the gas flow.Such grids or braces would be a suitable place to locate localnon-condensing gas extraction devices as this would be convenient interms of the overall structure of the design and the devices would beideally located for the removal of non-condensing gases.

Referring now to FIG. 16, this schematically illustrates a containmentvessel of a water-cooled nuclear reactor. The containment vessel has anouter wall 57 which is intended to prevent the release of gas to thelocal environment in the event of an excess pressure building up withinthe containment vessel. Housed within the containment vessel are a steamgenerator 58, a nuclear reactor 59 and a reactor coolant pump 60. Thesteam generator delivers steam to a turbine (not shown) through pipe 61and receives condensate from the turbine through pipe 62.

Spray heads 63 are located in an upper section of the containmentvessel. Large surface area trays 64 are provided on the floor of thecontainment vessel such that condensate formed within the containmentvessel will accumulate in those pools.

Were there to be a leakage of steam into the containment vessel, thiswould increase the pressure throughout the containment vessel. Indesigning such containment vessels, the current accepted practice is toconsider the gas mixture within the containment vessel as beinguniformly mixed. A heat transfer correlation is then applied to theentire surface area to determine the rate at which steam will condenseout, thereby reducing pressure within the containment vessel. The sizeof the containment vessel is made sufficiently large to ensure that thepressure never exceeds a predetermined maximum given the assumed designconstraints. As a result the containment vessels are extremely large,making it very difficult indeed to for example bury them in the ground,and the size of containment vessels is a significant factor in terms ofeconomic cost.

The understanding of condensation processes upon which the presentinvention relies makes it possible to reconsider the accepted teachingwith regard to the design of containment vessels. In particular, thenon-uniformity of the local air concentration means that the pressurewithin the containment vessel will be significantly lower than thatassumed in accordance with current design practice.

In a situation in which steam leaks into the containment vessel of FIG.16, steam would rise towards the top of the containment vessel due tobuoyancy forces arising from temperature differences between the steamand the air previously occupying all of the containment vessel. Thesteam would then condense and water droplets would begin to fall throughthe space defined within the containment vessels. If as shown in FIG. 16trays 64 are provided a horizontal surface of coolant would be definedwithin the trays once the trays were partially filled with condensate.The gas just above the surface of the coolant in the trays 64 would becooler than that at the top of the containment vessel and therefore theair concentration adjacent the coolant would be significantly higherthan at the top of the containment vessel. This would mean that thelocal heat transfer coefficient at the top of the containment vessel,which would determine the rate at which steam condenses out, would beenhanced at the top of the containment vessel. Given the exponentialnature of the relationship that is illustrated in FIG. 1, the pressuredeveloped within the containment vessel (which would of course beuniform throughout the vessel) would be lower than predicted by currentdesign predictions. Therefore future nuclear power containment vesselscould be considerably smaller than previously contemplated,significantly affecting the choices available to the designers of suchplant. Alternatively, maintaining conventional dimensions forcontainment vessels would result in increased safety margins.

The above discussion related to FIG. 16 assumes as in the case of thecurrently accepted design calculations that conditions within thecontainment vessel would result in heat transfer coefficients associatedwith free convection. In fact, in the event of a substantial leakage ofsteam the heat transfer regime would be forced convection and thereforestill further reductions in the adverse effects of high air fractionscould be expected.

In the arrangement of FIG. 16 the provision of schematically illustratedtrays 64 is indicated as providing a way to produce large surface areapools of condensate. These trays could for example be defined byrelatively small (in the vertical direction) formations on a flooracross which personnel normally simply walk. The effect of having a poolof condensate on the peak pressure within the containment vessel couldbe further enhanced by ensuing a very rapid formation of a pool ofcoolant rather than waiting for a pool of coolant to accumulate as aresult of steam condensation. For example, a tank of water could beprovided which would be rapidly discharged into a tray such as thatshown in FIG. 16 as soon as the pressure within the containment vesselexceeded a predetermined threshold.

Some embodiments of the present invention relate to condensers whichtypically are provided for the express purpose of converting a gas to acondensate. There are however some situations in which apparatus ismanufactured which is not typically referred to as a “condenser” butwhich relies upon condensation processes and it is the intention thatthe present invention should encompass such apparatus. For example,sterilisers are widely used which rely upon the injection of steam intoan enclosure to sterilise both the enclosure and articles placed withinthat enclosure. Typically, the enclosure is initially filled with airand a predetermined time is specified for purging the air by injectingsufficient steam so that the initial atmosphere of air within theenclosure at ambient pressure is converted to pressurised saturatedsteam. As steam is injected, the interior of the enclosure is pumped outor simply vented to atmosphere to remove air. In conventionalsterilisers, the air off-take pipe is located at the top of theenclosure. As a result the process of evacuating air from the enclosureis relatively slow. If the air off-take was located so as to extract gasfrom a region within the enclosure which is at a relatively lowtemperature, for example a region adjacent the bottom of the steriliseror adjacent a cooling device provided to produce a localised region oflow temperature, the rate at which air could be removed from thesteriliser would be significantly increased. This in turn would reducethe period for the purging process.

Thus in the case of a steriliser as discussed above, another embodimentof the invention applies the same principle to the extraction ofunwanted non-condensing gases as is applied in the case of condensersfor steam in electric power generation plant. The same principles can ofcourse be applied in apparatus other than sterilisers, for example heatexchangers in oil refineries or other chemical engineering plant wherethere is a need to remove non-condensing gases from a mixture ofcondensing and non-condensing gases.

While the inventions have been illustrated and described in detail inthe drawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiment has been shown and described and that allchanges and modifications that come within the spirit of the inventionare desired to be protected.

1. A method for minimizing the pressure within a containment vesselresulting from the release into the vessel of a pressurized gas whichwill condense to a liquid at the temperatures and pressures assumed toprevail within the containment vessel, comprising: providing acontainment vessel wherein a body of the liquid of large surface arearelative to the area of the vessel is provided in a lower portion of thevessel; and using the body of the liquid to establish a coolanteffective for producing a localized region of gas adjacent the coolant,the gas having a lower temperature than gas in other regions within thevessel and comprising a higher mass fraction of non-condensing gas,wherein the localized region of gas is effective to reduce the massfraction of non-condensing gas in an upper portion of the vessel.
 2. Acontainment vessel effective for containing a release into the vessel ofpressurized gas which will condense to a liquid at the temperatures andpressures assumed to prevail within the containment vessel, thecontainment vessel initially being filled with a gas or gases which willnot condense at the temperatures and pressures assumed to prevail withinthe containment vessel, and the containment vessel including a body ofliquid of large surface area relative to the area of the vessel forestablishing a coolant in a lower portion of the vessel so as to producea localized region of gas adjacent the coolant, the gas having a lowertemperature than gas in other regions within the vessel and comprising ahigher mass fraction of non-condensing gas, wherein the localized regionof gas is effective to reduce the mass fraction of non-condensing gas inan upper portion of the vessel.
 3. The containment vessel according toclaim 2, comprising at least one open tray arranged to collectcondensing liquid to form the said body of liquid.
 4. A containmentvessel according to claim 2, comprising means for releasing a storedvolume of the liquid into at least one open tray to form the said bodyof liquid.
 5. A containment vessel according to claim 4, comprisingmeans for sensing pressure within the containment vessel, and means forreleasing the stored volume of liquid in the event of the sensedpressure exceeding a predetermined threshold.
 6. A method for enhancingthe loss of heat from a mixture of gases to a surrounding containmentvessel to condense liquid from the gas to limit its pressure,comprising: providing a containment vessel having an interior and aninternal surface area, the containment vessel containing anon-condensing gas and surrounding an apparatus containing a heated andpressurized process fluid; after a portion of the process fluid isreleased into the interior of the containment vessel to produce amixture of condensing and non-condensing gas in the containment vessel,enhancing the loss of heat to the containment vessel to promote coolingof the condensing gas to produce a quantity of condensate and limit thepressure within the containment vessel; and collecting the condensate inone or more pools within a lower portion of the containment vessel, thecollected condensate having a surface area, the surface area of thecondensate being large relative to the internal surface area of thecontainment vessel; wherein the collected condensate is effective toproduce a localized region of gas adjacent the collected condensate, theregion of gas having a lower temperature than gas in other regionswithin the vessel and comprising a high mass fraction of non-condensinggas.